The critical difference between having six or seven cervical vertebrae

PZ Meyers in Seed Magazine:

Screenhunter_19_aug_05_1233Imagine a long-necked animal.

Most people will, I suspect, picture a giraffe. Other likely candidates are swans or long-necked dinosaurs or plesiosaurs; many vertebrates have evolved long, relatively flexible necks, the better to reach food that is otherwise out of reach, or is more easily captured with a mobile head on a flexible stalk.

Now picture the anatomical features that produce that long neck. That’s often more difficult if you haven’t poked around in comparative anatomy, but basically, the neck has a core structure of bony vertebrae stacked like spools along its length, each one separated from the other by a joint. This series of joints is what gives the neck its flexibility and, as you might guess, the more numerous the joints, the more flexible the structure would be. One simple question to ask is how many vertebrae are present in the neck of these various animals? It’s easy to count; just tally up the vertebrae from the base of the skull to the first vertebrae that bear ribs. The ones that lack ribs are the cervical (a fancy word for “neck”) vertebrae, and the first ones that have ribs are the thoracic (“chest”) vertebrae. Easy, but we get one surprising result.

More here.